In a known manner, starters comprise a means for protection against excessive current in the form, of one or more electric fuses and/or one or more circuit breakers. FIG. 1 thus shows an example of a starter motor 1 comprising a fuse 2 fitted in series between a supply terminal 3 of the electrical machine 1 and windings 4.1, 4.2, 4.3, 4.4 of the stator of the said motor.
The fuse is designed to blow in order to cut off the electrical supply of the motor when a predetermined maximum current passes through it. This makes it possible to avoid damaging the starter or its environment because of the heat produced by an excess current. Excess current of this type occurs in particular in the case when the rotation of the rotor of the motor is blocked.
When the machine is running unloaded or with a low load (i.e. when it is functioning at between approximately 0 and 25% of the nominal load), the unloaded speed of rotation of the rotor also gives rise to heating of the electrical machine which can damage the starter throughout the duration. This heating, which is causing particular by the mechanical friction between the brushes and the collector, occurs fur example during a phase of excess speed when the pinion of the starter which is driven by the thermal engine rotates faster than the shaft which is driven by the rotor of the starter. However, in these cases, the current which passes through the fuse can be too low to give rise to blowing of the fuse 2, even for a long duration.
A solution would therefore consist of selecting a fuse which can blow both when a no-load current passes through the fuse for a duration which exceeds a first predetermined duration, and when a current passes through the rotary electrical machine which has its rotor blocked for a duration which exceeds a second predetermined duration. Fuses of this type are however far more voluminous than a fuse of a standard size, which makes it virtually impossible to integrate them in the starter.
Document FR2977408 describes a compact thermal protection system 5 which makes it possible to cut off the electrical supply of the motor in the two aforementioned cases (in the case of blocking and in the case of functioning with a low load). For this purpose, this system 5 comprises a heat-sensitive unit and a contact blade which, after deformation of the heat-sensitive unit, can establish a short-circuit between a positive polarity to which the positive brushes 6.1, 6.2 are connected, and the casing 7 of the starter which is connected to the earth with the brushes 6.3 and 6.4. However, a thermal protection system of this type involves the creation of new parts, the fitting of which inside the brush cages is relatively complex to carry out.